Feet of altitude is a non-linear function of barometric pressure. If a linear differential is used to directly connect an aneroid to an indicator in an altimeter, an error of approximately 27 feet is built into the instrument in the operating range of from 28.10 to 31.00 inches of mercury. Therefore, a barometric pressure correction mechanism is required in order that an accurate indication is presented to an operator.
One method of correcting the output of the barometric pressure mechanism is to manually move an adjustable index on the altimeter dial to the feet of altitude corresponding to the ground level barometric pressure, thereby indicating ground level at the particular feet of altitude. In this method, the operator obtains information from an air data control station and thereafter makes the appropriate modification to the instrument through an input knob.
U.S. Pat. No. 3,218,862 discloses a correction apparatus. In this apparatus, the output of an aneroid is corrected by moving a helical gear set connected to an altitude indicator shaft through the engagement of a pin and driver gear connected to a helical gear associated with a cam surface. The cam surface engagement changes with changes in altitude to thereby provide the necessary correction for the altimeter display. However, such an arrangement employs many gears and is complicated.